A capacitor generally consists of two conducting metal plates separated by an insulating media (dielectric film) capable of storing electrical energy. The capacitor is formed by sandwiching the dielectric film between the two conducting metal plates. The film serves as an insulant which prevents the electrons from moving from one capacitor plate to the opposite plate. The maximum energy that can be safely stored in a particular capacitor is limited by the maximum electric field that the dielectric can withstand before it breaks.
The energy which can be stored by the film is proportional to the dielectric constant of the film and to the electrical breakdown of the dielectric as shown by the following equation.Energy density E=0.5·∈·∈0·BDV2,                where BDV is the breakdown voltage (in V/μm),        ∈ is the theoretical relative permittivity of the film, also named dielectric constant,        ∈0 is the absolute permittivity.        
The energy density of a capacitor containing a plastic film can be increased by increase of the relative permittivity. This is obtained by use of a polar polymeric material or by addition of a polar component such as polar polymers or additives such as ceramic particles in the polymeric film.
Capacitors in which a plastic film is used as the dielectric film with high relative permittivity are known in the art. For example, EP-A-0 039 214 describes capacitors in which the dielectric film comprises a vinylidene fluoride polymer, polycarbonate and/or a thermoplastic polyester.
U.S. Pat. No. 6,426,861 describes a plastic film having a high energy density. This film is made of a homogeneous mixture comprising at least one non-polar homopolymer and at least one polar or non-polar homopolymer. Examples of non-polar homopolymers are: polypropylene (PP), polyethylene naphthalate (PEN), polycarbonate (PC) and polyphenylene sulphide (PPS). An example of a polar polymer is polyvinylidene fluoride (PVDF). The homopolymers are blended and co-extruded with homogenization to form a melt-cast hybrid copolymer dielectric film. This document describes polymeric hybrid copolymers of PVDF and PP. It also describes a copolymer comprising PVDF and PP as one component and either PPS or PC or PET or PEN as a second component. The film disclosed in U.S. Pat. No. 6,426,861 is prepared from homogeneous solid-solution mixtures.
The energy density of a capacitor containing a plastic film can be increased by increase of the breakdown voltage of the plastic film.
EP-A-1 712 592 discloses a biaxially oriented film suitable as a film capacitor. This biaxially oriented film comprises a aromatic polyester (a) and a polyolefin (b) having a melting point of from 230 to 280° C., wherein the ratio of the polyolefin (b) is from 2 to 60% based on the entire weight of the film. The polyolefin is preferably a styrene polymer having a syndiotactic structure. It is preferred that the film does not have voids. The voids formed at the boundary between the aromatic polyester (a) forming the matrix phase and the polyolefin (b) forming the island phase. In case where voids are present, the film tends to be cut in the film stretching step. Further, as the film thickness is reduced, the portions of the voids result in defects which deteriorate the mechanical characteristic or deteriorate the withstand voltage characteristic. The voids can be eliminated by using a compatibilizing agent.
The present invention is directed to a plastic film having a high breakdown voltage. This film can be used as a dielectric film in a capacitor. It imparts a high energy density to the capacitor.
The present invention is also directed to a process for increasing the breakdown voltage of a plastic film by modification of the plastic film morphology.